In image compression methods such as MPEG-1, MPEG-2, MPEG-4 and H.264/MPEG-4 AVC, one picture is divided into macroblocks to encode an image. Then, the respective macroblocks are encoded using inter prediction or intra prediction.
In intra prediction, a current block of the picture is encoded not using a reference picture, but using values of reconstructed pixels spatially adjacent to the current block. An optimal prediction mode with little distortion is selected out of a plurality of intra prediction modes by comparing a prediction block generated using the adjacent pixel values with an original block. Then, using the selected intra prediction mode and the adjacent pixel values, prediction values of the current block are calculated. Differences between the prediction values and pixels values of the original current block are calculated and then encoded through transform coding, quantization and entropy coding. The intra prediction mode is also encoded.
According to H.264 standard, there are nine modes in 4×4 intra prediction. The nine modes are a vertical mode, a horizontal mode, a DC mode, a diagonal down-left mode, a diagonal down-right mode, a vertical right mode, a vertical left mode, a horizontal-up mode and a horizontal-down mode. One mode is selected among the nine modes to generate a prediction block of the current block, the mode information is transmitted to the decoder.
In HEVC standard under development, the number of intra prediction modes increases to 18 or 35, the size of coding unit lies between 8×8 and 128×128. The coding unit has similar purpose to the macroblock of H.264/AVC.
Accordingly, if the intra prediction mode is encoded using the same method of H.264/AVC, the coding efficiency deteriorates because the number of intra prediction modes is greater than that of H.264/AVC. Also, as the size of the coding unit increases and the number of intra prediction modes increases, quantization method and scanning method should be modified to enhance the coding efficiency.